In regard to conventional display devices, predetermined images can be displayed by utilizing a light emitted from a light source provided inside the device (internal light source; backlight unit).
Also, in recent years, in order to display an image with high front contrast while suppressing electric power consumption, there has been suggested a display device that uses an internal light source which can emit light with high parallelism that orthogonally intersects with the display surface of a display panel (hereinafter, may be referred to as collimated light) (hereinafter, the internal light source may be referred to as a collimated backlight).
With such a display device using a collimated backlight, light having high directionality can be made incident to various pixels that constitute the display panel, and therefore, light emitted from an internal light source can be efficiently emitted through the display surface side as image display light.
However, in such a display device using a collimated backlight, since directionality of image display light also increases, if the display device is used directly, there occurs a problem that the viewing angle becomes markedly narrowed.
Thus, in order to obtain a sufficient viewing angle even in the case of using a collimated backlight, an image display device in which an optical-diffusion film is laminated on the display surface side of the display panel has been disclosed (see, for example, Patent Documents 1 and 2).
That is, Patent Document 1 discloses a liquid crystal display device including a liquid crystal display panel, an optical-diffusion plate (light diffusion film) disposed on the image observing surface side of the liquid crystal display panel, and a backlight that causes a light for image observation to enter the liquid crystal display panel, characterized in that the optical-diffusion plate has a half-value angle of the output intensity distribution of 35° to 60° when parallel light is incident, and when the pixel pitch of the liquid crystal display panel is designated as p, and the thickness on the output side is designated as t, the light emitted by the backlight is a collimated light of which the half-value angle θ of the intensity distribution satisfies the formula:“0<θ≤tan−1(p/t)”.
Furthermore, regarding the optical-diffusion plate, there has been disclosed an optical-diffusion plate in which areas other than the light transmissive parts that are regularly or irregularly dispersed are covered with a black mask.
Furthermore, Patent Document 2 discloses a liquid crystal display device including a liquid crystal cell, polarizing plates disposed on both sides of the liquid crystal cell, an optical-diffusion element (light diffusion film element) provided on the outside of the polarizing plate on the viewing side, and a backlight unit provided on the outside of the polarizing plate on the opposite side of the viewing side, in which the backlight unit is a parallel light source apparatus which emits a collimated light having a luminance half-value angle of 3° to 35° toward the liquid crystal cell, and the ratio between the optical-diffusion half-value angle Fw (FD) of the optical-diffusion element and the luminance half-value angle Fw (BL) of the collimated light, Fw (BL)/Fw (FD), is 0.5 or less.
Furthermore, regarding the optical-diffusion element, an internal diffusion type optical-diffusion element containing light diffusible fine particles in the interior, or the like has been disclosed.